1. Field of the Invention
The present invention relates to a technique for preventing color misalignment in a color-image forming apparatus.
2. Description of the Related Art
In recent years, color-image forming apparatuses that adopt electrophotography, such as color printers and color copying machines have been required to output high-quality images.
As factors for determining the quality of an output image, there are misalignment of an image-writing position on a recording medium, recording accuracy typified by image expansion/contraction, and color misalignment, that is, the overlaying accuracy of color toner images which influences on the color of the image.
In particular, with the electrophotographic color-image forming apparatuses, degradation of recording accuracy and changes in color due to color misalignment are caused by environmental changes or variable factors of device components due to long use, thus degrading the quality of output images.
An example of the causes of such changes, for example, in an image forming apparatus that adopts an intermediate transfer belt as an endless belt, is speed fluctuations of the intermediate transfer belt.
Thus, for example, a method disclosed in Japanese Patent Application Laid-Open No. 01-142567 is used. Specifically, color toner patches are formed on an intermediate transfer belt, the positions of the toner patches are detected by a registration sensor, and the time at which the color-toner images are written to the intermediate transfer belt is changed using the detection results, thereby preventing color misalignment. Here, the toner patches are unfixed toner images for detecting color misalignment.
However, even if the known color misalignment correction using the registration sensor is executed, color misalignment occurs when the color toner images are actually transferred to a recording medium after the correction.
This is because the peripheral speed of the intermediate transfer belt when the positions of the toner patches on the intermediate transfer belt are detected by the registration sensor and the peripheral speed of the belt during actual-image formation differ. Here, generation of the difference in the peripheral speed of the intermediate transfer belt will be described in an orderly manner.
FIG. 11 is a diagram showing the state of a load applied on an intermediate transfer belt unit of a tandem-type color-image forming apparatus using a general intermediate transfer belt 30. In FIG. 11, to improve the transfer accuracy, the peripheral speed Vb of the intermediate transfer belt 30 is set about 0.5% or below higher than the peripheral speed Vd of photosensitive drums 26.
A belt driving torque T at that time is expressed by the following Eq. (1):T=Tb+μF×4  Eq. (1)where Tb is a torque that moves only the intermediate transfer belt 30 and μF is a frictional force that is generated due to the contact of the intermediate transfer belt 30 and the drums 26, where μ is the friction coefficient between the belt 30 and the drums 26, and F is a transfer pressure. Here, the contact means a state in which the intermediate transfer belt 30 and the photosensitive drums 26 are in contact to generate pressure, irrespective of the presence of a toner layer between the intermediate transfer belt 30 and photosensitive drums 26.
Next, as shown in FIG. 12, the belt driving torque T in which the drum peripheral speed Vd is intentionally set higher than the belt peripheral speed Vb is expressed by the following Eq. (2), and the belt driving torque T is decreased because the belt 30 is wound around the photosensitive drums 26.T=Tb−μF×4  Eq. (2)
Here, changes in torque after the belt 30 is driven from its halted state until it is halted again through image formation will be described with reference to Eq. (1).
First, when the friction coefficient μ between the belt 30 and the drums 26 is defined as the following two, changes in the torque T after the belt 30 is driven from its halted state until it is halted again through image formation are expressed by the following Eqs. (3) to (7). Changes in load torque applied on the belt 30 are shown in FIGS. 13 to 20.
In the drawings, reference numeral 26 denotes photosensitive drums, numeral 54 denotes developing rollers, numeral 52 denotes primary transfer rollers, and numeral 30 denotes an intermediate transfer belt. Reference character Y indicates yellow, character M indicates magenta, character C indicates cyan, and character Bk indicates black. Here, the friction coefficient μ between the belt and the drum is defined as the following two: a friction coefficient μ1 when there is no toner between the belt and the drum; and a friction coefficient μ2 when there is toner between the belt and the drum.T=Tb+μ1F×4  Eq. (3) (see FIG. 13)T=Tb+(μ1F×3+μ2F)  Eq. (4) (see FIG. 14)T=Tb+(μ1F×2+μ2F×2)  Eq. (5) (see FIG. 15)T=Tb+(μ1F+μ2F×3)  Eq. (6) (see FIG. 16)T=Tb+μ2F×4  Eq. (7) (see FIG. 17)
Hereinafter, see Eq. (6) (see FIG. 18)→Eq. (5) (see FIG. 19)→Eq. (4) (see FIG. 20)→Eq. (3) (see FIG. 13).
The two friction coefficients μ1 and μ2 generally have a relationship, μ1>μ2. The load (torque) applied to the belt is decreased when the developing roller 54 comes into contact therewith and is increased when the developing roller 54 comes out of contact therewith.
The mechanism of decreasing the load will be described in more detail. For example, FIG. 14 shows a state in which a developing roller 54Y is in contact with a photosensitive drum 26Y. At the point in time in FIG. 14, the toner on the developing roller 54Y adheres to the photosensitive drum 26Y as fogged toner, and thereafter, the fogged toner reaches a primary-transfer nip portion between the photosensitive drum 26Y and the intermediate transfer belt 30. Then, the load applied on the belt 30 due to the contact of the developing roller 54 is decreased owing to the action of the toner, so that the load on the entire belt is also decreased. As the process moves from FIG. 15 to FIG. 17, the total amount of the fogged toner that reaches the primary-transfer nip portion increases, and the load on the belt 30 decreases. On the other hand, as the process moves from FIG. 18 to FIG. 20, the developing rollers 54 are separated, and the fogged toner at the primary-transfer nip portion decreases, and, in contrast, the load on the intermediate transfer belt increases.
Next, with reference to the above description, a case in which toner patches are detected by the registration sensor will be described. A belt driving torque when toner patches on the belt 30 are detected by the registration sensor is constant in the state in Eq. (7), and the peripheral speed of the belt 30 is also constant. On the other hand, as has been described with reference to the foregoing Eqs. (3) to (7) and FIGS. 14 to 20, this state is different from a torque generation state (load generation state) directly after the start of image formation and directly before the completion of image formation.
On the other hand, it is known that the belt-drive transmission system constituted of a gear train for driving a belt is elastically deformed in proportion to stress generated from its load torque, as expressed by Hooke's law. Elastic deformation according to the generation of the load temporarily changes the transmission speed of the drive transmission system. In other words, it temporarily changes the peripheral speed of the belt. More specifically, the elastic deformation also has continuity, and therefore, also the position of the belt temporarily changes gradually due to the continuous elastic deformation. The temporary positional change of the belt causes fluctuations in the belt peripheral speed.
That is, the belt peripheral speed changes when the individual states in Eqs. (3) to (7) shift to the next states. For example, when the load torque applied on the belt changes from small to large, the belt peripheral speed slows down, and in contrast, when it changes from large to small, the belt peripheral speed increases. The fluctuations in belt peripheral speed here can also be regarded as following changes in belt position and can also be considered as changes in belt position due to temporary load fluctuations.
Even if toner patches on the belt are detected by the registration sensor, with no fluctuations in belt peripheral speed, and correction is made on the basis of the result, color misalignment (transfer position displacement) will occur because the belt peripheral speed fluctuates during actual image formation.
To eliminate the fluctuations in belt speed, there are the following three typical methods: first, eliminating elastic deformation of the belt-drive transmission system by increasing the rigidity thereof; secondly, eliminating fluctuations of the friction coefficient μ between the belt and the drum; and thirdly, executing image formation after the state of Eq. (7) has been achieved.
The first method will be described. In general, increasing the rigidity of the belt-drive transmission system can reduce the elastic deformation described above. For example, if the material of gears, which are elements of the drive transmission system, is changed from resin, such as polyacetal, to metal, such as brass, the rigidity can be increased. It has been confirmed by our experiment that speed fluctuations can be improved by increasing the rigidity using metal gears.
However, the metal gears have excessively high rigidity, which causes vibrations due to engagement, thus posing the adverse effect of applying the vibrations to an image. Moreover, since the metal gears are formed by cutting, the cost thereof is considerably higher than that of resin gears formed by injection molding, so that they are not practical.
The second method will be described. Theoretically, setting the friction coefficients μ1 and μ2 equal can reduce fluctuations of the friction coefficient μ. However, the surface layers of the present photosensitive drums are so smooth that they are prone to adhere to the belt, thus causing a significantly large frictional force. Although microscopic unevenness may be provided on the surfaces of the photosensitive drums to decrease contact areas, degradation of image quality can occur, so that it is not practical. Moreover, fluctuations in friction cannot be made zero because there is an attraction force due to transfer bias, in addition to the presence/absence of toner.
The third method will be described. The third method is technically feasible by turning ON/OFF the charging, developing, and transfer processes of the image-formation processing unit, which are the causes of generating load fluctuations, except when transferring a visible image from the photosensitive drums to the intermediate belt.
However, although this provides a high-quality image in which color misalignment is reduced, the ON/OFF of the charging, developing, and transfer processes of the processing unit is performed except when transferring a visible image from the photosensitive drums to the intermediate belt. This increases the processing time, such as charging and developing, thus decreasing the productivity of the apparatus. In other words, this has the problem of decreasing the life of the processing unit. In particular, when frequently printing a small number of pages, the influence is negligible. That is, this not only causes the user to frequently replace the processing unit but also increases its running cost.